1. Field of the Invention
The present invention relates to the construction of batteries and, more particularly, to battery constructions that allow energy to be withdrawn from the battery quickly.
2. State of the Art
The basic structure and operation of a conventional battery such as a lead-acid battery may be understood with reference to FIG. 1. The main elements of the battery are the anode, the cathode and the electrolyte. The anode, on discharge, supplies a flow of electrons, constituting an electric current, to an external device. The current powers the external device and returns to the cathode. Within the battery, the anode and the cathode are separated by the electrolyte. The electrolyte acts as an electrical insulator preventing electrons from moving between the anode and the cathode inside the battery. However, the electrolyte allows the movement of ions, the remainder of the atom or molecule from which the electron has been released. As electrons are released from the anode to the device, ions are released from the anode, cross through the electrolyte and are stored in the cathode. During the recharge process, ions are transferred through the electrolyte back to the anode.
In conventional batteries, the electrolyte is liquid. Liquid electrolytes effectively allow the movements of ions between the anode and the cathode. However, the use of liquid electrolytes has several disadvantages. In most batteries, the liquid electrolyte contributes a substantial portion of the battery's overall weight. In addition, most electrolytes are composed of dangerous chemicals, including acid. In the event of a short circuit or other physical damage to the battery, the liquid electrolyte is free to flow to the reaction site and produce a continuous chemical reaction. This reaction may result in excess heat or a gas discharge, and if not properly released, may be explosive. Although batteries are sealed when manufactured, leaks can occur through misuse or a breakdown of the battery's packaging over time, posing a significant safety risk.
Polymer electrolyte batteries represent a new battery technology that differs significantly from conventional battery technology and promises significantly improved performance. Generally speaking, a polymer electrolyte battery includes an anode, such as a lithium metal anode, a single-phase, flexible solid polymer electrolyte, and a cathode that stores lithium ions. Unlike the liquid electrolyte used in most batteries, the electrolyte is a solid--thereby substantially reducing the weight and volume of the battery. To date, however, solid electrolyte lithium batteries have not been available commercially because such batteries have operated effectively only at high temperatures.
In either conventional or lithium polymer batteries, each of the battery components has some electrical resistance associated with it. The sum of these electrical resistances along a current flow path through the battery constitutes the internal electrical resistance of the battery. Because of the internal electrical resistance of the battery, normal use of a battery produces resistive heating. Although substantial resistive heating does not arise in most typical applications, difficulties can arise when very large lithium batteries, such as those used for powering electric vehicles, are operated at high rates of discharge. In such instances, the battery encloses a large volume such that the center of the battery pack is, in effect, thermally isolated from the battery's environment. Furthermore, such batteries are required to provide high currents (700 amperes or more for a starting battery). Under these circumstances, significant heat may be generated by resistive heating. If no measures are taken to dissipate the generated heat, the heat may, in the extreme case, gradually build up to such a degree as to cause an explosion.
To cool batteries and to prevent significant heat from being generated by resistive heating, it has been proposed that heat dissipation for a battery be accomplished by bathing the battery in water and pumping the water through a radiator. Such a cooling method, though effective, is very cumbersome and expensive. It has also been proposed to use vents and fuses to reduce battery heating problems. However, such measures are effective only after a battery has started to "run away." By that time, it may be too late to prevent an explosion.
Resistive heating problems can also arise in smaller batteries which are misused. Misuse of a battery can include, for example, short circuiting the battery by connecting its terminals directly to another, connecting the battery backwards, or recharging the battery with an incompatible device. Resistive heating problems may also occur through no fault of the user. For example, if the anode and cathode are spaced very close together, a dendrite in the form of a small metal burr may form between the anode and the cathode, causing excessive heating.